feat(auth): implement GitHub OAuth to distribute API rate limits across authenticated users

[kanishka-2007-tech]

Description

This PR introduces GitHub OAuth authentication to reduce dependency on the application's global Personal Access Token (PAT) and prevent rate-limit exhaustion under high traffic.

Authenticated users can now sign in with GitHub, allowing dashboard-related GitHub API requests to be performed using their personal OAuth access token. The existing server PAT is retained as a fallback for unauthenticated users and README SVG generation endpoints.

Solution

Implemented GitHub OAuth authentication using NextAuth.

• Authentication Flow
• Added Sign in with GitHub functionality.
• Users can authenticate via GitHub OAuth.
• GitHub access tokens are stored securely in the session/JWT.
• Dashboard requests automatically use the authenticated user's token.
• Unauthenticated requests continue using the server PAT as a fallback.

Fixes #3679

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🛠️ Other (Bug fix, refactoring, docs)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

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📦 Next.js Bundle Size Report (Gzipped Sizes)

Asset	PR Size	Base Size	Difference	Status
static/chunks/app/api/Auth/Nextauthgithub/route-[hash].js	0.19 KB	-	+0.19 KB (++100%)	🆕 New

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3434.17 KB	3433.75 KB	+0.41 KB (+0.01%)
Total CSS	258.05 KB	258.05 KB	0 B